Added /files API test for queries with encoded sequences.
Added /files API test for queries with encoded sequences.

Due to a bug in the HTTP client in libprocess (MESOS-9168), the use of
percent-encoded sequences in paths did not work when using the HTTP
client in libprocess. This adds a test to ensure that the /files API
correctly handles these sequences in file paths.

Review: https://reviews.apache.org/r/68420

reneploetz/mesos,jpeach/mesos,chhsia0/mesos,jpeach/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,andschwa/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos,jpeach/mesos,Gilbert88/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,kaysoky/mesos,andschwa/mesos,jpeach/mesos,jpeach/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,andschwa/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos